1. Field of the Invention
The present invention relates to a swivel connection between two parts of an orthopedic technical aid, and more specifically is directed to a swivel connection having an adjustable, inherent articulation stability.
2. Description of the Related Art
Swivel connections are used, for example, in knee-joint prosthesis. Their flexion properties are characterized by a polycentric movement composed of translatory and rotatory components, which movement corresponds to the rolling of two curves on one another, whose respective point of contact constitutes the momentary pivot of the movement. Each of these so-called pole curves is assigned to one of the parts of the prosthesis connected to one another by means of the polycentric swivel connection. Depending on the layout of their basic geometry, such polycentric knee-joints afford--in addition to advantages in the sitting position--particular advantages during the stationary phase of walking (calculated from heel contact until lifting-off of the point of the foot) compared to monocentric, i.e., single-axis, knee-joint designs.
A monocentric knee-joint prosthesis, which is statically secured during standing by exclusive rearward movement of the single axis position behind the load line, becomes, under heel load at the start of the stationary phase of walking, unstable on account of the altered load direction and must consequently be secured against buckling by the amputee initiating a hip extension moment. The degree of this moment is reduced by a more rearward displacement of the central axis. However, such an increased rearward displacement of the central axis substantially increases the hip moment required to flex the knee joint at the end of the stance phase of walking, i.e., the physical effort required by the wearer to initiate the first walking stride by flexing of the knee joint from a standstill position is substantially increased. Consequently, the physical effort on the part of the amputee can only be lessened by reducing the rearward movement of the central axis, which in turn requires, on account of the associated loss of stability, compensatory measures such as, for example, the integration of additional stationary phase securing elements in the form of mechanical brakes or hydraulic dampers.
In contrast, at a correspondingly high position of the momentary pivot of the extension position, a polycentric knee-joint prosthesis can have, not only during standing but also under heel load at the start of the stationary phase of walking, such a marked inherent stability that it is secure in the extended position without any hip extension moment. With a corresponding design, a further advantage can be obtained at the end of the stationary phase, at which point the natural walking pattern, for harmonic transition to the subsequent swing phase, provides for the initiation of a knee-bending under anterior foot load. This cannot be imitated by a prosthesis with a rearward-displaced, monocentric knee-joint, since the average amputee cannot apply the necessary hip flexion moment for an indefinite period of time. In contrast, the flexion of a prosthesis equipped with a polycentric knee-joint of the above-mentioned design under anterior foot load at the end of the stationary phase requires a far smaller hip flexion moment, which the average amputee can apply without tiring. However, these advantages can only be achieved if in each case the optimal position can be adjusted for the momentary pivot of the extension position.
Notwithstanding the advantages of a polycentric knee-joint prosthesis having the above-mentioned functional features, there is generally one disadvantage: its pronounced inherent stability in the extension position becomes lost at small flexion angles. It has therefore not been possible to design an artificial leg that affords an amputee the possibility of initiating, during an approximation of a natural walking pattern, an elastically spring-cushioned and/or damped knee flexion without loss of stability at the start of the stationary phase of walking under heel load in order to garner effective shock absorption while further improving the walking pattern and ergonomics in order to render the vertical movement of the body's center of gravity during the entire course of the stationary phase more balanced and more harmonic.
In the field of prosthetics, and in particular, orthopedics, the term below-knee prosthesis or "BK-Prosthesis" is used to indicate a type of prosthesis used for patients who have an amputation below the natural knee joint. The term through-knee prosthesis or "TK Prosthesis" is used to indicate a type of prosthesis used for patients who have an amputation just through or almost through the natural knee joint. The term above-knee prosthesis or "AK-Prosthesis" is used to indicate a type of prosthesis used for patients who have an amputation of the thigh, above the natural knee joint. The terms "hip-disarticulation prosthesis" or "hemi-pelvectomy prosthesis" is used to indicate prosthesis used for patients who have amputations just through, near or above the natural hip joint.